1. Field of the Invention
The present invention relates to an organic light emitting device (OLED) in which an oxazole-, thiazole-, or imidazole-fused phenanthroline molecule is used in an emissive layer, a charge transport layer, and/or a charge blocking layer.
2. Description of the Related Art
OLEDs are commercially attractive for various display applications due to their high efficiency, low driving voltage, wide color range, light weight, simple device fabrication and potential low cost. Great efforts have been made to develop effective materials for use in these devices.
OLEDs typically comprise an emissive layer sandwiched between a transparent anode, such as transparent indium tin oxide (ITO), and a metal cathode, such as Mg, Al, Ag or their alloys. When a bias is applied across the electrodes, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the emissive layer. The holes and the electrons radiatively combine in the emissive layer and emit light.
To improve the power efficiency of an OLED, it is frequently desirable to enhance charge injection at the electrode interface. Hole transport layers and electron transport layers may be added adjacent the respective electrodes to facilitate charge transfer. In some instances, the emissive layer is located within the hole transport or electron transport layer.
Improved performance can be obtained if blocking layers are provided to block against the injection of either holes or electrons from the adjoining layer and their subsequent escape from the device.
A layer may have combined functionalities. For example, an electron transport layer may also function as a hole blocking layer.
In addition, it is possible to form the various layers from a host material doped with another material designed to achieve the desired effect of the layer (for example, to achieve a charge transport effect, a charge blocking effect, or an emissive effect).
In an electro-luminescent OLED, the luminescence observed is fluorescent. Fluorescence is understood to mean luminescent emission occurring only during the period during which an excitation (such as driving voltage) is applied, and is created mostly, if not exclusively, by relaxation from the singlet state.
In an electro-phosphorescent OLED, in which a phosphorescent dye is utilized, both singlet state relaxation and triplet state relaxation contribute to emission, and emission persists after the period of excitation.
Because of consumer expectations of good efficiency, long lifetime and pure color for OLEDs, a need exists for the development of effective materials for use in these devices.